October 4, 2011

Carl Sagan wrote that if the Earth were the size of a globe our atmosphere at that scale would be about as thin as a single sheet of paper. Yet the air we breathe is so fundamental to life that even the tiniest changes in the composition of our air could cause cascading climate change on a global scale. While carbon dioxide is present in only trace amounts, scientific consensus indicates at a change from 292 parts per million today to 380 parts per million by the next century could spell disaster for human civilization through rising sea levels, changes in established wind and ocean currents, melting ice caps and desertification associated with the overall average trend of global warming. And so atmospheric regulation is essential to the continuing survival of all life; this biogeochemical process is mainly controlled by single-celled organisms.

Our atmosphere on Earth is mostly made of nitrogen, but it’s also about 20% oxygen. Oxygen is a corrosive gas that is dangerous under high concentrations because of how reactive it is with other chemicals. But this concentration of oxygen has been by no means stable over the last four-and-a-half billion years; oxygen levels in our atmosphere have been virtually non-existent until about half a billion years ago, when the so-called Great Oxidiation Event (GOE) took place. The GOE released huge plume of oxygen into the sky over the course of a few million years, radically altering the compostition of life on Earth and giving rise to the ancestors of the eukaryotes. Early life on Earth was anaerobic, meaning that it could function without oxygen. While aerobic animal life takes in oxygen and burns it during metabolism to create carbon dioxide and energy as waste, anaerobic life uses a myriad of metabolic pathways to produce energy, reducing molten iron, acetate, sulfate, hydrogen gas, or other inorganic molecules to produce their energy.

It wasn’t until the rise of cyanobacteria that any appreciable oxygen could be produced. These cyanobacteria are blue-green algae that performed photosynthesis by taking in sunlight, carbon dioxide and water to grow. One byproduct of this reaction was oxygen gas. The early Earth environment was highly reducing, meaning that it would readily absorb any oxygen and quickly oxidize something in the environment. Substrate like iron (III) dissolved in the water would readily oxidize and become iron (II) oxide, which was insoluble in water and would sink to the bottom of primordial seas. We find these banded iron deposits around the world and they are a ready source of the iron we use in modern manufacturing. The presence of banded iron formations would indicate the presence of oxygen being produced by cyanobacteria at the time, so we can reasonable assume that aerobic photosynthesis was going on around 2.1 billion years ago. In fact, cyanobacteria were so pervasive on Earth that their combined exhalations of oxygen radically altered the composition of air.

Areios’s atmosphere is mostly nitrogen, like the Earth’s atmosphere, but the outgassing of volcanoes and the rapid destruction and creation of crust means that oxygen is less abundant in the atmosphere, which has profound implications for the development of animal life. Combine this with the later start for photosynthesis, and this means that aerobic life doesn’t appear until about 12 billion years into Areios’ existence, yet this kind of more complex life persists for over 3 billion years before the surface temperature gets too hot for photosynthesis to maintain itself permanently. Fifteen billion years after creation, the planet’s atmosphere undergoes another profound change. As Hemera gets brighter, the atmosphere would start to slump off and this would alleviate some of the heat that gets trapped in the Areiosan atmosphere. Eventually the atmosphere becomes so thin with carbon dioxide that there isn’t enough CO2 to fuel photosynthesis and plants would die off en-masse. This drop in carbon dioxide would eliminate the greenhouse effect on Areios, and in turn this massive die-off would incite the next ratcheting up of carbon dioxide, which would in turn lead to a positive temperature feedback loop. Oceans would boil over until the last life left on the planet would paradoxically resemble the earliest life; a halophilic thermophile. Eventually, even this hardy creature wouldn’t be able to survive Areios would once again be a world sterile of all life. Temperature would still rise, though, and would boil the carbon dioxide out of the carbonate rocks in the crust, causing a runaway greenhouse effect like the one that we see on Venus. Sadly, this is the fate of all terrestrial planets as their parent star grows old; the same fate awaits our own planet earth in the coming eons.

Long after the Earth's atmosphere boils away, our Sun will evolve into a Red giant star.

June 11, 2011

Check out this latest press release from the NASA Science News webpage; the Voyager probes are beaming back some very interesting information on what its like at the far reaches of our own solar system.

February 8, 2011

In our solar system, the Moon is the most massive satellite in the solar system relative to the size of the planet that it orbits. Because the Moon is so big compared to other satellites, its gravity tugs our oceans into areas of high and low tides, depending on the interactions of the Sun, the Moon and the Earth. Areios has three small moons, which makes their interaction with life on the planet interesting. We experience areas of high tide to the side facing the moon, and when the moon is at its farthest approach, we experience areas of low tide. If we didn’t have a moon, the Sun’s gravity on Earth would still cause some tides to occur, but not nearly as pronounced. Not only does the moon interact with the Earth’s oceans, the moon’s orbit has been slowing down the rotation of the Earth. The moon’s orbit has been growing wider and wider, too. In the next few billion years, the moon will fall into a closer orbit around the earth and get torn apart by the Earth’s gravity.

Our planet Areios has three moons, two of which were created from an impact event similar to the one that created Earth’s moon. The middle moon was an asteroid captured during the Late Heavy Bombardment era. Of Areios’ three moons, the innermost and outermost moons will wander out of Areios’ gravitational pull one day, but the middle moon will collide with the planet’s Roche Limit at some point, forming a ring of debris that could get dislodged and rain fragments down down Areios, spelling doom for anything unfortunate enough to be caught in the crossfire down below. Earth’s moon stabilizes the earth orbit, keeping the axial tilt of the planet in check, unlike Mars. Because Mars has two smaller moons, its axial tilt is out of control and the climate fluctuates wildly with each season without a massive moon to keep it in check. Areios experiences seasonal fluctuations more akin to Mars than Earth because its three moons all orbit with a different period and angle, only coming into alignment once every few hundred years. This would cause wild changes in weather leading to bitterly cold era of winter and longer blazing hot era of summer. These mood swings in climate could be challenging for any life on the planet to adjust to and such an unpredictable climate might lead to a higher rate of extinction during some epochs, and this would free up niches in the environment for new evolutionary forms to exploit, essentially increasing the turnover rate for species, so to speak.

On Earth, Milankovitch cycles caused by interactions with the gravitational fields of the other astronomical objects in the solar system with the Earth triggers regular perturbations in the Earth’s orbit, but these mild events happen like clockwork that a Serbian astronomer deduced their recurrence. Precession is any change in rotational axis or orbital path of a planet and both the Earth and Areios go through regular patterns of precession over geologic time. The Earth’s axis completes one full cycle of precession the orientation of Earth’s axis of rotation shifts slightly approximately every 26,000 years, creating a wobbling effect like a spinning top. At the same time the elliptical orbit rotates more slowly because of Jupiter or Saturn‘s gravity tugging at the Earth‘s orbit. The combined effect of the two precessions leads to a 21,000-year period between the seasons and the orbit. The angle between the Earth’s rotational axis and a perpendicular plane is call obliquity (or axial tilt), and the Earth moves from 22.1 degrees to 24.5 degrees and back again on a 41,000-year cycle. Areios goes through similar orbital changes but the most pronounced processional change is the obliquity in its rotational axis. Its axial tilt will vary up to a couple degrees every 35,000 years or so. Because the axial tilt is so extreme, Areios would experience different seasons and more extreme changes in weather.

Areios is very peculiar in that it rotates on its axis like Uranus, with the poles tilted very nearly onto the plane of revolution it has around Hemera. This gives it polar ice caps around the East and West instead of North and South. This strange arrangement was brought up briefly in the discussion of precession, but the impact that formed Areios’ two moons are also responsible for its off-kilter tilt of about 90 degrees. That means that the northern hemisphere is in constant light for weeks or months on end at one point and the other one is in constant darkness. Half an orbit later the roles are reversed. And halfway between those times, the rotational axis is perpendicular to the Sun’s direction, making day and night alternate in a way similar to what the Earth experiences at equinox. Any organism living on the planet would have to adapt with wildly-dramatic seasons that would vary from frigid temperatures in the winter to sweltering temperatures in the summers. Some creatures might adapt by burrowing into the ground to avoid the climate extremes and some may go into hibernation. We’ll discuss how animals survive such a harsh environment, but for now it will suffice to say that because these creatures have to live in an environment far different from what we find on Earth, a whole new set of morphological adaptations would be present on Areios to help them survive a solid month of perpetual sunlight or darkness.

February 1, 2011

Alkyoneus would dominate the Areiosan night sky as one of the brightest objects visible to the naked eye. This is because Alkyoneus is a planet more massive than Jupiter and its girth carries a strong gravitational field causing perturbations or disruptions in the orbits of its neighboring worlds. This gas giant can act as a shield, diverting dangerous comets and asteroids away from Areios, or the massive planet’s gravity can act as a plow, pushing the rocky debris left from the solar system’s formation onto a collision course with the planet through planetary migration. Areios’ water, for instance, came in part from planetoids left over from the accretion stage of Areios’ formation. Rocky chunks of planet collided and the heat from friction melted those bits together, forming ever bigger planetesimals that lead to full-fledged planets. Alkyoneus was such a big planet that its gravity would keep hold of gases swirling around and it enveloped so much mass that the atmosphere keep building until it was a gas giant planet. Gas giants like Jupiter or Alkyoneus have a mostly-silicate core and layers of gas thousands of kilometers thick that girdle the planet. The composition of Alkyoneus and its atmosphere reflects how close it was to Hemera when it formed. The current theory for the formation of our solar system is explained in the nebular hypothesis; the solar system started out as a swirling ball of gas with a dim star still forming and sucking in mass at the center. As particle grains grew by colliding with each other in a process called accretion, the velocity and direction that these particles were whipping around in became more or less averaged out, so all of the material flattened out into a disk with everything basically moving about on the same plane, in the same direction, at roughly the same speed. A few collisions might redirect the path of individual moons or planets, but eventually most of the matter in the solar system would get incorporated into a planet. The exception to this in our solar system has to do with Jupiter’s gravity; its mighty pull tore apart anything orbiting around where a fifth terrestrial planet could be expected and kept it from forming anything bigger than the asteroid Ceres. In Areios’ solar system, there are a significant number of asteroids in a belt between Areios and Alkyoneus, representing a mass about equal to Earth’s moon.

The asteroids in Earth’s solar system are categorized by their composition. C-type asteroids are carbaceous (carbon-rich) and s-type asteroids are silicatious (silicate-rich), with more c-type asteroids farther from earth and s-type asteroids closer to us. Planets that form close to the star would have much less volatile content; volatiles are things like water or carbon dioxide that would boil away early on in the planet’s formation from the heat given off by its star. Hemera may be dimmer than our Sun, but anything caught too close to its radiation would melt. This is why terrestrial planets are found closer to the Sun in our solar system than the gas giants; as the solar system formed, the Sun wasn’t a fully-functional main sequence star and the composition of the nebula swirling around it was homogeneous. So as the Sun condensed and started to heat up, it melted the frozen ices closest to the Sun, leaving behind silicates and metals, which have a higher boiling point than ice. A star’s luminosity decreases with distance, so the radiation that reaches the middle to outer portions of the solar system wouldn’t melt the ices as much, so ices and volatile chemicals would make up a greater proportion of the planets farther out.

Once the planets formed, Alkyoneus’ gravity started to hold on to more gas vented from the crust or sucked in from the nebula surrounding the planet and the weight of the atmosphere crushed any hydrogen gas closer to the core into a metallic form. On the periodic table hydrogen is above lithium and the alkali metals, suggesting that because they’re in the same period, they would have similar properties. Hydrogen only behaves like a metal under the most extreme pressures, but when it’s condensed into a metallic form, the hydrogen nuclei form a tightly packed grid and the electrons are no longer confined to any individual proton, like in a sea of electrons. From the metallic hydrogen core, Alkyoneus’ atmosphere has bands of cold dense gases and other streams of hotter and faster moving gases. Alkyoneus’ atmosphere is made up of hydrogen, helium, and traces of methane, ammonia, cyanide, carbon monoxide and noble gases. There is more than enough organic material to start life, but there are concerns about buoyancy; any life has to stay in a layer of the atmosphere that’s not too hot or too cold, so altitude within the clouds has to be carefully maintained because if a creature rises too high or sinks too low, they could freeze, broil, or be torn apart by fierce winds. To maintain the right depth, a creature would need an organ like an air bladder (akin to the swim bladder on a fish that keeps it from sinking or floating to the top of the water), but primitive life couldn’t have a complex feature like this, so if a single-celled organism can’t have a swim bladder, it’s hard to imagine a complex creature ever being able to evolve.

A depiction of the gas giant Alkyoneus and its seven moons, the Alkyonides

January 25, 2011

Areios orbits Hemera somewhere between where Mercury and Venus would be in our Solar System; at first, Areios lies just within its habitability zone where liquid water could exist on the surface, and because Hemera initially output less light when Areios was forming, it seems unlikely that life could arise on this frigid planet right from the start. On Earth, life may have formed just as soon as the crust solidified and the oceans condensed from the atmosphere, yet our planet would have been cooler than it is now were it not for the greenhouse effect. This early arrival for life suggests that life could be common in the universe if it can be spawned on a habitable planet so early in its formation. Because Areios is on the edge of the habitable zone, it may take some time to warm before it can be habitable for life. The habitability zone for life will change over time as Hemera evolves; Areios is positioned in the very outer habitable zone near the beginning of Hemera’s life and by the end of its main sequence stage, Areios is only just tucked inside the inner edge of that habitable zone. For the 38 billion years or so that Hemera is in the main sequence stage, Areios is within the habitable zone for 30 billion of those years. For the first and last four billion years of Hemera’s evolution, Areios will be sterilized of all life, first because of freezing temperatures early on and then because of the boiling temperatures near the end of Hemera’s life. As Areios’ surface temperature gets pushed hotter, Areios will have to shed more and more of its thick atmosphere like a jacket to cool off until its atmosphere is too thin to support liquid water on its surface and the oceans boil away. But more on the evolution of a habitable planet later…

Areios formed from the collision of planetesimals billions of years ago; these violent interactions also created two out of three of its moons and a third moon was captured later during a period of asteroid and comet bombardment. When the cataclysms of the planet formation ended, Areios was a still-molten ball of rock with a swirling ring of debris that would later become its moons. Areios is more massive than Earth and contains a bigger mantle and a thinner crust. The importance of this will be revealed later on, but this distinction is not trivial when it comes to the potential for life on Areios. Because of Areios’ girth, the planet would take longer to cool and the internal portions of the planet would stay hotter for longer because the core is wrapped in a much thicker insulating blanket of mantle. This early planet would soon cool on the outside, though, and a process called differentiation would occur; Areios was at first a well-mixed sphere or magma, but it began to cool and settle. The crust formed like a skin like a bowl of soup left to cool; this outermost layer is only a few kilometers thick, but covers the entire planet and serves as the palette for the thin veneer of life that is to come. The crust covers the mantle of the planet, which makes up the bulk of Areios’ mass. The mantle is a made of melted rock kept solid by the intense pressure coming down on it; unlike Jules Verne’s Journey to the Center of the Earth, there are no caverns or caves in the mantle because this solid rock can flow like a liquid and would quickly fill any void beneath the planet, despite the fact that pressures make the molten material behave like solid rock.

This is a depiction of what a planet like Areios would look like early in its formation.

As Areios formed, there were three processes that kept generating the internal heat of the mantle; the kinetic energy of impacts, differentiation, and radiogenic heating. During the formation of Earth, there was a period called the Late Heavy Bombardment where more comet and asteroid impacts struck the planet, boiling the oceans and melting the crust for a couple hundred million years before the rain of fire subsided. When these objects struck the Earth, their gravitational potential energy as they fell to the earth was converted to kinetic energy in the form of heat. Areios experienced a similar event to the late heavy bombardment for a couple of hundred million years after the planet formed and for a while the kinetic energy from those impacts kept heating the planet, but once those impacts subsided, the planet cooled enough to form crust and the oceans. Areios started out as a homogeneous ball of magma, but slowly the heavier metals started to settle in the core of the planet. As these denser materials sank into the mantle, their potential energy was converted into kinetic energy until the planet differentiated into the three distinct layers of the crust, mantle and core. Once these three layers were fully formed, the planet no longer generated heat by differentiation. The final and only ongoing way the interior of the planet generates heat is through radiogenic heating. When the planet formed, it incorporated some heavier unstable elements like thorium and uranium. Over time, these elements would decay into lighter elements like potassium or lead; this radioactive decay would release energy in the form of heat that keeps the internal parts of Areios hot. This Late Heavy Bombardment era for Areios would deliver water to the planet and later determined how much of Areios will be covered in lakes and oceans.

January 17, 2011

Hemera’s solar system formed when a molecular gas cloud called a nebula compacted into its center and formed a denser mass of dust. As material got attracted to the center of this cloud, it released heat through transforming gravity’s potential energy into the kinetic energy of motion. When gravitational forces began the collapse, the cloud’s slower rotation picked up under the conservation of angular momentum. As the center of this cloud started to attract more mass, the gravitational pull that this mass became stronger and the mass was able to pull into more mass and release more energy by this transformation. This process went on until the gravity of this gas ball became so massive that it started to crush the hydrogen in the center until it has dense enough and hot enough to form helium. Once this object was hot enough to undergo nuclear fusion of hydrogen into helium, it became a main sequence star.

As the solar system was forming, grains of rock and gas were swirling in an orbit around Hemera; these bits would collide frequently and sometimes these particles would smash into each other and form larger particulates. Eventually, these collisions would cause the orbits of all of the particles to roughly take on an average speed and direction, so the solar system would flatten out into a plane where all of the chunks of planetesimals would orbit around in. Eventually, those tiny grains of dust accreted into large rocky protoplanets, satellites ranging from the size of a continent to roughly the size of Mars. These planetesimals would collide with one another and form the planets we see now. Areios experienced one extraordinary collision that formed two of its three moons. A collision with a mercury-sized object called Dione created two silica-rich moons that orbit around the planet. The impact sent over 100 moonlet fragments into orbit around the planet and eventually formed three moons; one of them was cast out into deep space after six weeks. These two remaining moons, Otus and Ephialtes, are revolving farther and farther from the planet each year and this slows down the orbit of Areios over billions of years. Eventually, both of these moons will escape Areios’ gravitational pull and roll off into space. A third moon Eriboea was snagged in a later capture event that nestled it in between the first and third moons created by the impact event. Spiraling inward retrograde to the first and third moons, this moon will one day collide with Areios’ Roche limit and be ground up in a planetary ring. When that debris gets dislodged by gravity and some rains down on the planet, it would spell doom for anything unlucky enough to be alive at the time.

In Hemera’s solar system, we see a similar process of accretion that leads to two planets being formed; one is a massive gas planet that orbits far out into the Solar System; Alkyoneus, more massive than Jupiter, captured seven planetesimals into a lunar orbit before they could be smashed in a collision. These seven moons called the Alkynonides would large enough to be planets with satellites of their own, were they not captured by the gas giant’s formidable gravity. These planetesimals have plate tectonics, volcanism and a thin atmosphere, but orbit too far from the Sun to have liquid water on their surface. The radiation blasted from the gas giant irradiates anything on the surface, making it difficult for life to start. With no oceans or thick atmosphere to protect life from the harmful radiation, and it seems unlikely that life would spawn around Alkyoneus. The Alkyonides are in a part of deep space that’s far from the habitable zone where liquid water can exist on the surface of a planet. But liquid water could exist on these moons underneath the crust, warmed by the volcanism of the internal heat generated within the planet.

Our final stop is a tiny speck out in the far reaches of the solar system; this lonely planet is much like our Pluto, covered in a layer of ice along with its own little frozen satellite. Perses and the satellite Hekate were once part of the Alkyonides, but early on in the formation of the solar system, Perses and its satellite were dislodged from orbit and were lost in space until they finally reached a stable orbit at the very fringes of the solar system. Perses was the smallest of the Alkyonides, but still managed to snare a comet for a moon; when it was flung into a orbit at the outer regions of space, a comet settled in an orbit around Perses. Although Perses was once tectonically and volcanically active, it no longer produces a magnetic field or has any significant atmosphere. Now it just hangs out in the dead of space, the last stop in the solar system before the great beyond of the stars.

January 10, 2011

We’re going to focus on a star that’s just within this galactic habitable zone; it’s on the outer edge of the galaxy and is too dim to see from the center of the galactic habitable zone. Our star is called Hemera and it is even punier than our mediocre Sun; while the Sun fuses lighter elements into calcium and gives off a healthy yellow light Hemera is smaller, less compact and looks feverish with its deep orange-red glow. This hue is because Hemera has less mass than our star so gravity doesn’t push down on it as hard, and it’s less dense than the Sun because it burns its fuel more slowly, and with less luminosity. This means that our star will stay in the main sequence stage of its life for longer than the Sun, which is good news for our creatures on this planet orbiting Hemera; it gives them more time to evolve into a form that could one day jump ship before Hemera goes nova.

For our star with a mass of about three-fourths the mass of the Sun, life could potentially live on Areios around Hemera as a main sequence star for about 30.5 billion years before Hemera will go defunct. At 70% of our Sun’s mass, it would only shine at about a quarter of the brightness of our Sun.24 This could be problematic for life if the planet isn’t situated close to its star to stay warm. Once a planet gets too close to its star, though, it may become tidally-locked, like our moon is to the Earth, with the same side perennially pointing towards the surface of Hemera. One side of the world would boil and the other side would freeze. A planet can overcome this with an atmosphere thick enough to circulate heat and keep one side of the planet’s atmosphere from boiling and the other side from freezing.7 Curiously, the planet would be divided into three distinct zones; one of perpetual light, one of perpetual darkness, and one of perpetual twilight between the two opposing hemispheres of light and dark. 6 Astrobiologist Nancy Kiang suspects that plants on this world would be jet black to absorb as much of the dim sunlight as possible around a red dwarf star.8

Note that the Sun-like Star on the right is the most moassive and has the widest habitable zone of the three stars.

For a planet orbiting a red dwarf star, the only way to keep warm is to orbit in this tidally-locked configuration that would keep one side of the planet always pointing to the star and one side always pointing away, but a new model of planet habitability suggests that planets orbiting a red dwarf may be host to habitable planets. Red dwarf stars can exist in the main sequence stage for tens of billions or maybe even a 100 billion years, certainly longer than the age of the universe so far.5 This would mean that a red dwarf star could be habitable to life for tens of billions of years, much longer than on Earth or Areios. Also, because these stars don’t burn out at the rate of more massive stars, they make up a greater percentage of the stars in the night sky. Astronomers estimate as many as 75% percent of the stars in the universe could be red dwarf stars.10 This makes planets within the habitability zone of red dwarves a priority for astronomers looking for habitable planets, if only technology were sensitive enough to detect such dim stars and their planets. However, there are an abundance of them for planet hunters to find.

It should be mentioned that a star will get brighter with age. Over the lifetime of a star, the luminosity increases as mass decreases and gravity pushes down on the star with more intensity. As a star loses fuel, gravity pushes it harder towards a center point which concentrates the remaining fuel it has left into a more efficient sphere that burns harder to correct for the now overpowering gravity to bring the star back to equilibrium. When stars are just forming, they release high-energy radiation that would sterilize the surface of any planet too close; this activity lessens with time, but it would leave the surface of a planet uninhabitable at first. Low-mass red dwarf stars undergo these shifts in luminosity called flares to a much greater extent than a star like Hemera.9 Once Hemera gets over this early phase, it will produce less ultraviolet radiation than our sun, and generate more infrared and visible light, making it potentially safer for life on the surface. Our Sun has grown about 33% brighter over the last 4.5 billion years.3 At first Areios may be less habitable for life because it’s so cold orbiting around a dim star, but over time that star would get hotter and brighter.

The surface temperature of a planet isn’t solely dependant on the amount of radiation it receives from its star, either. Surface temperature also depends on how effectively the atmosphere can trap heat in a phenomena called the greenhouse effect. Without the greenhouse effect, Earth would be much cooler and while carbon dioxide levels have fluctuated over geologic time, there is ubiquitous consent that rising carbon dioxide levels are causing a more pronounced greenhouse effect. The greenhouse effect occurs when certain greenhouse gases like water vapor, carbon dioxide, and methane allow shorter wavelength light to pass through the atmosphere, but traps longer wavelength infrared light, to keep it from escaping.1 Infrared light heats up the planet enough on Earth to keep the tropics from freezing, but when humans pumped more carbon dioxide into the atmosphere from burning fossil fuels, it trapped more heat and caused a series of climate changes that we have yet to figure out what the long-term impact of that decision will truly be. The same mechanism that intervenes to maximize the surface temperature on our planet wouldn’t be as pronounced on a world like Areios with a hyperactive plate tectonic system to scrub carbon dioxide from the planet. The greenhouse effect that is causing widespread climate change on Earth could keep Areios warm enough to form liquid water on the surface.

January 3, 2011

The most distant object we’ve sent through our solar system is the Voyager 1 probe. Launched on September 5, 1977 it has since reached the very edge of our solar system and will soon pass out of the heliopause, or the point where our sun’s solar wind is weaker than the interstellar wind of other stars, effectively drawing the end of our solar system. Well after its original mission was completed, Voyager 1 continues to collect data on our Sun and its perplexing heliosphere.1 After traveling for 33 years, this probe has only made it 0.002 light years from our planet, but it has flown by everything in our solar system. It’s from this perspective that we begin our story of an alien solar system; from the outgoing point of view of an interstellar probe, the star we are about to discuss looks no bigger or brighter than any other star in the night sky, (and it’s not even visible to the naked eye) but it holds the little-known distinction of an abode for life.

Stars can be classified by their brightness, their size, their composition, and their ability to undergo fusion. Scientists also classify stars based on their spectral characteristics, or what color the star looks like as it burns, which is based on what kinds of heavier elements are getting fused in the star’s interior, which in turn is determined by how hot the star is. We can graph these stars in a diagram that compares the mass of a star with its luminosity. This is called the Hertzsprung-Russell diagram and it shows a band of stars on in the center sloping down and to the right that signifies the stability strip of main sequence stars.2 This strip shows the correlation between the mass of a star and the brightness and temperature with which it burns. Stars generate their heat and light through the process of fusing elements together to form heavier elements called nucleosynthesis. A star maintains its integrity by balancing the inward push of gravity with the outward thrust of nuclear fusion. When the star runs out of fuel to continue fusion, the inward push of gravity overcomes the weakened thrust of fusion and the star collapses, getting hotter as the gravitational collapse causes potential energy to be released, concentrating the heat of the star into a much smaller volume. Eventually, that star releases all of that energy in a nova.3 In our star, mostly hydrogen fuses together under immense temperature and pressure to form helium; in other stars hotter than our Sun, heavier elements can be formed in greater quantities and at a faster rate than our scrawny little star. Stars are the furnaces that create all the chemical elements in the universe and when these gas balls burn out and explode in a nova, releasing the heavy elements that will one day make up the satellites around another solar system. The more massive the star, the bigger the explosion and these supernovas release every known element out into the universe.

When the universe began, there was only hydrogen and helium with a smattering of lithium and beryllium in the smallest trace amounts. The first generation planets to form in the afterglow of the Big Bang were only made of hydrogen and helium, more akin to the gas giants in Earth’s solar system rather than a terrestrial planet like Areios. It wasn’t until the first generation stars went supernova that we see any satellite resembling a rocky planet because heavier metals that make up the bulk of our world weren’t forged until a supernova explosion sent those products out into the stars. The second generation planets could have been made of rocky material and may have been habitable for life once elements like carbon, nitrogen, and oxygen were created. Areas of a galaxy with more hydrogen and helium tend to create more bigger stars that create more violent supernova explosions.

Because of this, NGC 772 has areas of more star-building compared to other areas with less star-building and this means that certain regions of NGC 772 is more habitable than other regions, which is conceptualized by the galactic habitable zone.4 The GHZ of NGC 772 looks like a donut; the very center of our galaxy is too close to a super massive black hole at the center of the galaxy; that black hole will spit out dangerous radiation and any planet too close would get irradiated with high-energy particles that could tear apart a cell. Any planet too close to the center would be sterilized. Any star in the center of the galaxy would also be closer to more massive stars and would be subjected to more devastating supernova explosions that might hinder the development of life. Too far out into the edge of the galaxy and there’s not enough gas to form stars and planets and any satellite out there would not have enough metal to build a habitable planet. It’s somewhere in between the outer and inner portions of our galaxy that we find the right space conducive for building habitable planet and stars. This concept of a “goldilocks” zone that’s just right for life will come up again later in our discussion a planet’s orbit around a star. But more on that later…

September 7, 2010

If its nighttime where you’re reading this, and if you’re blessed to live in an area that isn’t swamped by light pollution, you might be able to see the stars in our night sky. Just west of Taurus and just east of Pisces is the constellation Aries. Hiding between those stars is the galaxy dubbed NGC 772, an unbarred spiral galaxy 130 million light years away. Because it’s 130 million light years away, we can only get a picture of what this galaxy looked like 130 million years ago. What if with an especially-powerful telescope we saw a planet forming in some star in this galaxy? By the time that light got into our telescopes, that solar system today would be remarkably different from the image we see. Our view of these deep space objects are so out-of-date that we have virtually no way of knowing what is going on in these places in real-time.

So what if we had a telescope powerful enough to detect a certain dim star in that galaxy? Assuming that the star we were looking at was lined up in just the right way, we might be able to detect a planet orbiting in that system. And we point our telescope at this star and we find that periodically, that star gets even dimmer at a regular interval. We conclude that this dimming is caused by a planet that makes a transit between our telescope and that star, blocking some of our light and it passes in front. Astronomers call this the transit method of detection, and with this method, planet hunters have found hundreds of planets, some of them are even rocky like our planet Earth. For years, we check the regular dimming of this star and we take note of where and when the planet passes in front of its star. This occurs on regular intervals and we use it to determine the volume of the planet based on how much light is being blocked. This method called transit photometry; our current technology only allows us to find planets within a certain distance of the star it orbits, though, but it’s effective at seeking out smaller-mass planets like the rocky terrestrial worlds in our own inner solar system.

From there, we could figure out the planet’s mass based on its gravitational pull and radial velocity measurements to quickly we deduce from all of this data that our planet is made of rock, slightly more massive than earth, and orbiting close to its star. Because we have a powerful instrument that could detect the composition of this planet’s atmosphere, we learn to our surprise that this planet’s atmosphere is nitrogen-based with carbon dioxide, water vapor and other gases. Imagine our surprise when we discover this planet is similar to Earth in so many ways that it could be habitable to life as we know it.

Planet habitability is the measure of a planet’s potential to sustain life, which is determined by a wide range of factors. For instance, where a star is located in the galaxy can determine how likely it is life will arise on a planet orbiting that star. Stars located closer to the galactic center are more likely to be irradiated by the cosmic rays emitted by galaxy’s massive core. Also, stars form much denser clusters in the center of our galaxy, so it would be likely that these planets would get caught in the supernova explosions by their neighboring stars. This would mean that planets would get doused with high-energy radiation that would rip apart organic molecules and make it difficult for life to form. The Earth is located on the outer edge of the Milky Way galaxy, so we’re in less danger of this radiation bombardment. However, as we look at farther from the center of the galaxy, elements heavier than hydrogen and helium get rarer because a lower density of stars mean supernovas happen more infrequently and fewer metals get formed by nucleosynthesis, so building a planet made out of anything other than hydrogen gas is harder at the edge of our galaxy. Assuming a star is located in the area in between either extreme, it could be host to a habitable planet, one that is not only conducive to life as we know, but one that stays habitable long enough to spawn life.

Check out our next post when we take a closer look at a Solar system in the Arietis galactic habitable zone, home to a planet that will serve as the subject of our upcoming thought experiment.